Convert TestPlatform.sln to slnx format

[nohwnd]

Replace the legacy .sln solution file with the new .slnx XML-based format introduced in .NET 9+. The .slnx format is cleaner, easier to diff, and the recommended format going forward.

Changes

• Add \TestPlatform.slnx\ generated via \dotnet sln migrate\
• Remove \TestPlatform.sln\ (Arcade SDK does not support both coexisting)
• Update \CONTRIBUTING.md\ to reference .slnx\
• Update \scripts/test.sh\ solution variable
• Update \DataCollectorAttachmentsProcessorsFactoryTests.cs\ repo root detection

Verification

• Build succeeds with 0 warnings, 0 errors via \�uild.cmd\

Closes #15543